Highly aligned electrospun nanofibers were fabricated using a field-controllable electrospinning process. The technique involved a cylindrical electrode connected to a spinning nozzle to stabilize the initial jet, and a field-controllable electrode generating an alternating current (AC) electric field fixed with a rotating collector. Aligned polycaprolactone (PCL) micro/nanofibers were prepared successfully using this process. Due to two electrostatic forces, the Coulombic force and the field-induced torque due to dipole-dipole interaction, the PCL micro/nanofibers were highly stretched in the direction of the electric field; the alignment depended on the applied frequency of the electrode. Wide-angle X-ray diffraction measurement was used to observe the crystallinity and molecular orientation of the electrospun fiber mats. As using the field-controllable rotating collector, the molecular orientation of PCL was improved relative to that of the normal electrospinning process. The oriented electrospun PCL fibers exhibited an increased storage modulus compared to conventionally fabricated electrospun fibers. In addition, the average fiber diameter was reduced and the distribution was narrower compared to those fabricated by the conventional electrospinning process.